Method for diagnosing insulation degradation in underground cable

ABSTRACT

The invention provides a method of on-line diagnosis of insulation degradation in a portion of underground cable including measuring the current flow at first and second locations of the cable utilizing an optical current sensor at one of a number of current frequencies, determining the current flow differential at that frequency, and analyzing the results of the current flow differential determination according to a corrosion test, a partial discharge analysis, a harmonic current analysis or a tangent delta diagnostic.

This application claims the benefit of provisional application No.60/109,545 filed Nov. 23, 1998.

TECHNICAL FIELD

The invention relates to methods of testing insulated underground powercables for degradation of the insulation, and more particularly toon-line diagnostic methods.

Background Art

Electrical power is typically distributed throughout the world usinginsulated underground power cables. Such power cables typically consistof a conductive core of a bundle of conducting strands, surrounded by asemi-conducting shield layer, an insulation layer, a secondsemi-conducting shield layer, a layer of metallic tape or helicalconcentric neutral conducting strands, and a polymeric jacket or sheath.The insulation on such power cables has a typical lifetime of about 30to 40 years. However various factors can cause premature degradation ofthe insulation and resultant failure of the cable. One typical form ofdegradation is “water treeing”. Polymer insulation absorbs moisture overtime, and collections of moisture in the power cable insulation underelectrical stress are referred to as “water trees”. Such water trees cancause capacitive leakage current distortion from the central conductorwhich over time and under certain electrical conditions can lead toelectrical treeing and ultimately complete failure of the cable.Similarly, partial discharges and electrical treeing can occur due tothe presence of imperfections such as cavities or contaminants orparticles in the insulation layer within the cable or its accessories.Such treeing can cause premature failure of the power cables.

It is important therefore for utilities to be able to test power cablesfor degradation to allow replacement prior to failure and to permit anorderly replacement schedule. Preferably such testing would be done“live”, that is without de-energising the cable prior to testing. Whilethe Japanese power system which uses ungrounded delta connected cableshas permitted live testing because of its different groundingarrangement, North American power cables and in most other countriesworld-wide, use grounded, star connected cables that have required inthe past that the cable be de-energising for testing. There is thereforea need for a diagnostic method which permits on-line, non-destructivelive diagnostics in the North American and world-wide cable systems.While various on and off line diagnostics are known, they generallyrequire different methods of detecting the current leakage, andconsequently it is not possible to carry out multiple diagnostics usingthe same measurement set-up.

Disclosure of Invention

The invention provides a method of on-line diagnosis of insulationdegradation in a portion of underground cable comprising:

i) measuring the current flow at first and second locations of the cableutilizing an optical current sensor at one of a plurality of currentfrequencies:

ii) determining the current flow differential at said one frequency;

iii) analysing the results of the current flow differentialdetermination according to one or more of the following diagnosticanalyses to assess the need for cable replacement:

a) a corrosion test on the cable to ensure the safety of conductingtests thereon;

b) a partial discharge analysis; and

c) one of the following two diagnostics, depending on the type ofinsulation material:

i) a harmonic current analysis if the insulation layer is cross-linkedpolyethylene or ethylene propylene rubber, or

ii) a tangent delta or loss angle diagnostic if the insulation layer ispaper insulated lead covered or ethylene propylene rubber.

BRIEF DESCRIPTION OF DRAWINGS

In drawings which illustrate a preferred embodiment of the invention:

FIG. 1 is a schematic illustration of an underground cable system onwhich the invention is performed; and

FIG. 2 is a cross-section taken along lines A—A of FIG. 1.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1 and 2, a standard underground power cable (notto scale), designated as 10, carries an electric current, typically arated maximum 200/600 amperes at 50/60 hertz. The cable consists of astranded conductor 12, concentric neutral wires (also referred to as“ground wires” or “drain wires”) 16 which are grounded, polymerinsulation layer 14, polymer semiconductor shields 17 and insulatingjacket 19. Water trees 18 have developed in insulation layer 14 usuallyat voids or contaminants 11 or at protrusions 13, 15.

Extremely accurate optical current sensors 22 and 24, which may be fibreoptic, slab or crystal, are now available to measure the flow of currentin the cable. Such sensors must have a resolution of 5×10⁻⁶ amperes.Optic sensors of this general type are manufactured currently by 3M Corpand others. A suitable optical current sensor is slipped on to the cableends or wrapped around or clipped over the cable. The light within thefibre, slab or crystal is perturbed by the electromagnetic fieldproduced by the current in the conductor and can be calibrated tomeasure the current in the conductor with sufficient accuracy andlinearity. The output from the optical current sensors is communicatedto central processing unit 20 which calculates the difference betweenthe cable conductor current flows at 22 and 24 at different frequencies.Readings are taken at different frequencies according to the diagnosticperformed, whether 50/60 hertz, 150/180 hertz, greater than or equal to50 kilohertz, or direct current. The differential is then processed, anddisplayed on laptop computer 21. The data is analyzed by the laptopsoftware and recommendations are made on cable replacement based on thatanalysis. To reduce the effects of background noise, time averaging ofthe data is carried out over a twenty minute period. Multiple cablesample analyses are possible at one time increasing the utilisationfactor of the diagnostic. The present cable diagnostic method can thuscombine at least four technologies into the one on-line unit therebyincreasing the diagnostic capabilities.

Corrosion of the concentric neutral wires 16 occurs when the concentricneutral wires are exposed to ground without the protection of aninsulating jacket 19. Corrosion will also occur on a damaged orpunctured jacket that allows the current to pass from the concentricneutral wires to ground. Corrosion currents may continue to flow untilthe concentric neutral wires are completely eroded away and unsafevoltages can occur on the cable. At this point the cable is a safetyhazard and must be replaced immediately as specified by the ElectricalSafety Code. As a first step in the diagnostic procedure therefore it isnormally advisable to ensure that the concentric neutral wires areintact and to detect any corrosion of the concentric neutral wires, toavoid a safety hazard. The corrosion test may be conducted either at 60hertz or dc (direct current). The method of carrying out a corrosiontest currently is described in the following publications: F. Escalante,U. Bertocci et al., “Development of In-situ Techniques for the Detectionof Corrosion of Copper Concentric Neutrals of Electric Cables inUnderground Environments” IEEE Transactions on Power Apparatus andSystems, Vol. PAS-101, No. 7, July 1982, p. 2061; K. G. Compton,“Corrosion of Concentric Neutrals of Buried URD Cable”, IEEETransactions on Power Apparatus and Systems, Vol. PAS-101, No. 6, June1982, p. 1651; “Protecting Copper Underground”, EPRI Journal, March1983, p. 20. In the present invention, the test is carried out using theoptical current sensor by positioning the sensors 22, 24 on theconcentric neutral wires on the outside of the insulation layer 14.Existing methods use Time Domain Reflectrometry, Ground PenetratingRadar, Step Potentials, resistance measurements, and ground gradients,but none of the existing methods utilize an optical current sensor.

If the concentric neutral wires are not corroded to an extent whichmight be a hazard, then the diagnostic method can be continued.

It has been found that the following diagnostic can then be carried outto analyze the deterioration of the cable based on the measurementstaken by sensors 22, 24.

1. Partial Discharge Analysis

Regardless of the type of material used for insulation layer 14, apartial discharge analysis at 50 kilohertz or greater is conducted tohelp locate discrete problem areas made evident by partial dischargeactivity within the cable insulation system including the cableaccessories (terminations, separable connectors and splices) and thecable insulation. Imminent failures can be established and forcedoutages avoided through this analysis. Previously this analysis has beendone using a “capacitance tap” or by installing a metal electrode toprovide capacitive coupling to the cable and its accessories. Priormethods of carrying out a partial discharge test are described in thefollowing publications: R. Bartnikas, “Power Factor and Corona DischargeTest” in Power Cable Engineering, Chapter IX, p. 263 (SandfordEducational Press, 1987); R. Bartnikas, “Corona Measurement andInterpretation” in Engineering Dielectrics Vol. 1, R. Bartnikas and E.J. McMahon, editors, STP 669 (ASTM Press, Philadelphia, 1979); L. A.El-Zeftawy and T. D. Eish “Analytical and Experimental Investigation onCable Insulation”, Modelling Simulation and Control A: General PhysicsMatter and Waves, Electrical and Electronics Engineering Vol. 27, No. 3,1990 pp 403-408; R. D. Naybour, “Examination of the Breakdown in New andAged Polyethylene Cables”, Proceedings on the 3rd Conference onConduction and Breakdown in Solid Dielectrics, 1989, Trondheim, Norway,pp 61-65; T. Tanaka, M. Watanabe and K. Yatsuka, “Detecting theBreakdown Causes of LPE Cable by the PPD Method”, Revue Generale del'Electricite, No. 1, January 1992, pp. 35-39; L. A. Dissado and J. C.Fothergill, “Partial Discharge and Free Volume Breakdown”, ElectricalDegradation and Breakdown in Polymers, Ch. 13, Peter Peregrinus Ltd. Inthe present invention, however, the diagnostic is conducted by directlymeasuring the current flow at sensors 22, 24 and analysing thedifferential.

A second diagnostic is also conducted, depending on the material of theinsulation layer 14. If the insulation layer is cross-linkedpolyethylene or EPR (ethylene propylene rubber), then a harmonic currentanalysis is conducted. If the insulation layer is paper insulated leadcovered or EPR then a tangent delta or loss angle diagnostic is used.

2(A). Harmonic Current Analysis (150/180 Hz)

Whereas there is a healthy leakage current of about 10 mA at 60 hertz, abadly deteriorated cable will show a leakage current (differential) ofabout 300 microamps at 180 hertz (150 hertz outside North America). 50to 200 microamps shows there is moderate deterioration. The method ofcarrying out the harmonic current analysis is described in the followingpublications: K. Hirotsu, K. Hosoe et al., “Development of Hot-lineDiagnosis Method for XLPE Cables by Measurement of Harmonic Current” inProceedings of the Symposium on Electrical Insulation Materials, Osaka,Japan, September 1994, Vol. 26, pp. 455-458: J. Densley, “Aging andDiagnostics in Extruded Insulation for Power Cables” in Proceedings ofthe 6th International Conference on Conduction and Breakdown in SolidDielectrics, Jun. 22-25, 1998, Vasteras, Sweden; J. Densley, “Didactic”,IEEE ICC Minutes, November 1997, St. Petersburg, Fla. This method hasbeen utilized in Japan on ungrounded cable systems but has not been usedon North American systems.

2(B). Tangent Delta or Loss Angle Diagnostic

The 50/60 hertz leakage current (capacitive current) is measured andcompared to the applied voltage of each cable to determine thedissipation factor (tangent δ) of the cable insulation (measured at 60hertz in North America, 50 hertz elsewhere). Comparative agingassessment between cables can be achieved by evaluation of dissipationfactor values. For new cables, values less than 1.0e⁻⁰⁴ can be expectedwhereas aged cables will have dissipation factors close to 1.0e⁻⁰¹.Cable replacement priority can be easily determined from this analysis.The method of carrying out the tangent delta test is described in thefollowing publications: R. Bartnikas, “Power Factor and Corona DischargeTest” in Power Cable Engineering, Chapter IX, p. 263 (SandfordEducational Press, 1987). Again, this method has not been carried outusing optical current sensors.

In this way a single current sensor set up can be used to carry outmultiple diagnostics. By carrying out two or more of the diagnostics amore complete and accurate analysis of the deterioration of the cablecan be obtained. Further, by conducting all of the tests using a singlemeasuring equipment set-up and single diagnostic unit, instead of fourdifferent diagnostic devices, the speed of analysis is increased andcost of the diagnostics greatly reduced. The fact that all thediagnostics can be conducted on-line reduces the switching costsincurred in de-energizing the cable. Further, switching surge damage canoccur to the cable insulation during the switching required to preparethe cable for off-line diagnostics as well as during re-energizing whenreconnecting the cables for service.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. Accordingly, the scope of the invention is to beconstrued in accordance with the substance defined by the followingclaims.

What is claimed is:
 1. A method of diagnosis of insulation degradationin a portion of underground electric distribution cable comprising thesteps of: i) measuring the current flow at first and second locations ofthe cable utilizing a current sensor at a plurality of currentfrequencies or direct current; ii) determining the current flowdifference at each said frequency or direct current; iii) analysing theresults of the current flow difference determination according to one ormore of the following diagnostic analyses to assess the need for cablereplacement: i) a harmonic current analysis, or ii) a tangent delta orloss angle diagnostic; wherein such method is carried out on-line,without disconnection of said cable from the electric distributioncircuit, the current flow is measured at said first and second locationsof the cable utilizing first and second optical current sensors at aplurality of current frequencies or direct current, and the results ofthe current flow difference determination are analyzed according to oneor more of the following diagnostic analyses to assess the need forcable replacement: a) a corrosion test on the cable to ensure the safetyof conducting tests thereon by measuring the difference in the directcurrent in the concentric neutral wires of said cable between said firstand second locations; and, if said concentric neutral wires are notcorroded to an extent which might be a hazard; b) a partial dischargeanalysis by determining whether partial discharge activity is presentusing existing methods of analyzing partial discharge applied to saidcurrent difference between said first and second locations; and c) oneof the following diagnostics, depending on the type of insulationmaterial: i) a harmonic current analysis if the insulation layer iscross-linked polyethylene, or ii) a tangent delta or loss anglediagnostic if the insulation layer is paper insulated lead covered, oriii) either or both of a harmonic current analysis and a tangent deltaor loss angle diagnostic if the insulation layer is ethylene propylenerubber.
 2. The method of claim 1 wherein said corrosion test is carriedout at 50/60 hertz or direct current.
 3. The method of claim 1 whereinsaid partial discharge analysis is carried out at 50 kilohertz orgreater.
 4. The method of claim 1 wherein said harmonic current analysisis carried out at 150/180 hertz.
 5. The method of claim 1 wherein saidtangent delta or loss angle diagnostic is carried out at 50 or 60 hertz.6. The method of claim 1 wherein said optical current sensor is a fibreoptical current sensor.
 7. The method of claim 1 wherein said opticalcurrent sensor is a slab-type optical current sensor.
 8. The method ofclaim 1 wherein said optical current sensor is a crystal-type opticalcurrent sensor.
 9. The method of claim 1 wherein said corrosion test isfirst carried out, followed, if said concentric neutral wires are notcorroded to an extent which might be a hazard, by said partial dischargeanalysis and one or both of either said harmonic current analysis orsaid tangent delta analysis.